In addition to propulsion, an aircraft's main engines provide shaft power to drive accessories such as electric generators and pumps, and fresh, pressurized air for the aircraft's environmental control system. This air is used to cool or heat, ventilate, and pressurize the aircraft cabin. The extraction of pressurized air and shaft power from the main engines is referred to as parasitic loss, and must be compensated for by increasing fuel consumption. Not surprisingly, aircraft and aircraft engine designers are continuously working on reducing these parasitic losses, and hence reducing the fuel consumption of the main engines.
When the aircraft is on the ground and the main engines are shutdown, many aircraft employ an Auxiliary Power Unit, (APU). An APU is a small gas turbine engine comprising one or more compressors and a turbine mounted on a shaft. A combustor is disposed between the compressor and the turbine. During operation, the compressor supplies pressurized air to the ECS, and the turbine provides shaft horsepower to drive accessories such as an electric generator. Recent technological advances now enable APUs to start and operate during an in flight emergency such as a main engine shutdown. However, under normal in flight operating conditions the burden of providing electricity and pressurized air still falls on the main engines.
One approach to reducing the parasitic losses of the main engines is to combine an APU and ECS into one system and have the APU's compressor continuously provide pressurized air to the ECS. Such systems are disclosed in Cronin; U.S. Pat. Nos. 4,494,372 and 4,684,081. Although these systems eliminate the parasitic loss due the extraction of pressurized air from the main engines, they still require shaft horsepower from the main engines to drive electric generators. Christoff, U.S. Pat. No. 4,503,666 discloses another approach which is to combine an APU and ECS into one system and have the APU drive an electric generator. However, in both the Cronin and Christoff systems the APU consumes fuel. Therefore, though the fuel consumption of the main engines is reduced, the overall fuel consumption of the aircraft is not necessarily reduced.
Accordingly, there is a need for an ECS that can provide both conditioned air and electricity to the aircraft without an increase in fuel consumption.